I. Field of the Invention
This invention relates to a compound-superconducting coil, and more particularly to a compound-superconducting coil wherein a compound-superconducting wire is held in a pipe, and a coolant such as liquid helium is forced through said pipe.
II. Description of the Prior Art:
To date, a superconducting coil constructed by winding a superconducting wire, containing a compound-superconducting material such as Nb.sub.3 Sn, has been put to various applications, for example, superconducting coil for nuclear fusion, NMR coil for research, and strong magnetic field coil for determining properties of matter.
The conventional compound-superconducting coil has been mainly constructed by simply winding a compound-superconducting wire into a coil. The superconducting coil thus formed has been put to practical use by dipping it in a coolant such as liquid helium and applying a magnetic field to the surrounding of said coil. Though possessed of an excellent superconducting property, the conventional superconducting coil is accompanied with the drawback that it has little mechanical strength and close care should be taken in working it into a coil, and cracks easily develop in such a coil during operation.
In "Selection of a cryostabilized Nb.sub.3 Sn conductor cooling system for the Large Coil Program, 7th Smp. on Eng. Problems of Fusion Research", J. W. H. Chi et al describes a superconducting coil constructed by holding a compound-superconducting wire in a tube and forcing a coolant through the tube by means of a pump. However this proposed coil has the drawback that when the coil is continuously subjected to great bending strains, the critical current sharply drops. When the critical current of the coil stands at less than 80% of that observed during the strain-free state of the wire, the superconducting wire is damaged and fails to retrieve a superconducting property even after the current load is released. Thus, it has been impossible to manufacture a superconducting coil with a small diameter which withstands great bending strains.